An optical system for quantitative measurement of cardiac muscle myoglobin oxygen saturation in the presence of hemoglobin and cytochrome interference is proposed. All of these components exhibit oxygen dependent changes in spectral properties in the visible wavelength region. Myoglobin oxygen saturation will be distinguished independent of hemoglobin saturation or volume and cytochrome oxidation using advanced statistical analysis of the multiwavelength signals. Myoglobin oxygen saturation measurements of solutions with known composition will be used to calibrate a multiwavelength diode array system. Controlled measurements of the paced dog heart will be used to transfer the in vitro calibration to in vivo determination of the myoglobin oxygen saturation. The system will then be used to investigate the dynamics of tissue oxygen in the working dog heart. The multispectral analysis approach will provide a powerful new tool to study oxygen in heart.